I got this forward from a friend and we figured it out and then googled to see what others were saying. Believe me you can find framewise analysis, physiological theories, to plain wonder.

Wanna know how to see the girl spin both ways? Just look at it from 2 different angles. Either tilt the monitor or move your head sideways. It is just a trick of optical angle.

My friend and I were looking at it and both of us always seem to see the exact opposite and then suddenly it occured to us to just turn the monitor towards the other guy, We discovered at that instant our perception of how the girl moved changed. It seems to be a trick with the angles.

But It still could have something to do with which half of the brain gets to process it first. Or it could be due to the differential delay in the propagation of the optical impulse to wherever it gets processed.

Dont know what they call it. Hey we are just amateurs.

P.S. Incase you dont find the original post, you can check it out on my picasa album.